Boron-boron quadruple bond in Li3B2- and Li4B2 clusters.

Homopolar quadruple bonding in first row p-block elements is expected due to the presence of four valence orbitals accessible for bonding. Although quadruple bonding in C2 has been proposed, no such proposal exists for B2. Here we report the unprecedented B-B quadruple bonding in Li3B2- and Li4B2 clusters based on high level theoretical calculations. The quadruple bonding is omnipresent in the global minimum, its nearest energy isomer and the transition states connecting them. Various bonding analyses reveal the unprecedented nature of the BB quadruple bonding interaction.